For all systematic velocity measurements,
floats were exclusively used, viz., surface floats, double floats, and
loaded rods. Their advantages and disadvantages had been fully discussed
in the detailed treatise "Roorkee Hydraulic Experiments"--1881. They
measured only "forward velocity," the practically useful part of the
actual velocity. The motion of water, even when tranquil to the eye, was
found to be technically "unsteady;" it was inferred that there is no
definite velocity at any point, and that the velocity varies everywhere
largely, both in direction and in magnitude. The average of, say, fifty
forward velocity measurements at any one point was pretty constant, so
that there must be probably average steady motion. Hence average forward
velocity measurements would be the only ones of much practical use. To
obtain these would be tedious and costly, and special arrangements would
be required to obviate the effects of a change in the state of water,
which often occurred in a long experiment, as when velocities at many
points were wanted.
As to surface-slope its measurement--from nearly 600 trials--was found
to be such a delicate operation that the result would be of doubtful
utility. This would affect the application of all formulas into which it
entered. The water surface was ascertained, on the average of its
oscillations, to be sensibly level across, not convex, as supposed by
some writers.
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